Mars
is a prominent red "star" in our skies this summer
because it is making a rare and relatively close
pass to the Earth. Mars is the next planet beyond
Earth and completes one orbit around the Sun in
687 days. The Earth moves faster in its orbit around
the Sun, and roughly every 26 months the planets
pass close together and then the Earth overtakes
our celestial neighbor. When this happens, Mars
is said to be in "opposition" because it appears
to be on the opposite side of the sky from the Sun.
During these times, Mars becomes bright and relatively
large in telescopic views. However, because the
orbit of Mars is quite elliptical, some close approaches
are closer than others. At the best possible oppositions,
which occur every 15 to 17 years or so, Mars approaches
within only 35 million miles (56 million km) of
Earth, while in the poorest oppositions Mars is
over 63 million miles (100 million km) away. The
almost factor of two increase in the apparent size
of Mars at the best oppositions explains why telescopic
observers concentrate their observations around
those times.

The 2001 opposition of Mars is the
best in terms of the apparent size and brightness
of Mars in the sky since 1988. It is also easily
the best since HST started observing Mars in 1990.
Mars came within 43 million miles (68 million km)
of Earth in late June 2001, when it appeared as
a bright red -2.2 magnitude "star" in the constellation
Scorpius. Mars will continue to shine brightly in
this part of the sky for the rest of the summer.

This close pass of Mars provided
an unprecedented opportunity for high spatial resolution
telescopic observations from HST. At its closest,
HST was able to resolve features as small as about
10 miles (15 km) apart on the Martian surface. This
is by far the best resolution ever obtained from
an Earth-based telescope, and is the same kind of
resolution that was being obtained from Mars spacecraft
flybys and orbiters in the 1960s and 1970s! Additionally,
HST's high quality digital cameras can obtain images
at scientifically important wavelengths that are
not being studied by any of the past or present
Mars space missions, thus filling an important gap
in color coverage and substantially enhancing the
scientific return of those missions. HST is, in
effect, another NASA mission to Mars--just in a
very, very high orbit!

Here we present
a color composite of some of the images obtained
by HST on June 26, 2001, when Mars was very near
its closest approach point to Earth. The composite
consists of images obtained through blue
(410 nm), green (502 nm), and red (673 nm) filters,
as well as several others with the Planetary
Camera detector on the WFPC2 instrument. The colors
have been balanced to provide an approximate representation
of "true color" as would be seen through a backyard
telescope, although the contrast of the clouds,
hazes, and ices visible primarily at blue wavelengths
have been enhanced slightly for better visibility.

The Hubble images and resolution
at this opposition are spectacular. A number of
interesting surface and atmospheric phenomena are
visible, and they remind us that Mars, like the
Earth, is a dynamic and ever-changing world. To
first order, the surface is divided into bright
reddish and darker grayish regions. The reddish
color is caused by the presence of oxidized iron
minerals (rust) on the Martian surface. Smaller,
bluer/whiter areas are seen near the poles (top
and bottom in this color composite) and the limbs
(left and right sides). These kinds of markings
have been observed in roughly this same pattern
for at least the last 300 years, since serious telescopic
observations of Mars began. However, the markings
are known to change with time. Bright regions have
been seen to darken, and dark regions to brighten,
and many changes have been observed especially along
the boundaries between bright and dark regions.

These kinds of changes are caused
by the constant movement of bright, fine-grained
dust across the surface, carried by the Martian
winds from place to place. Two of these dust storms
have in fact been caught in the act in these new
HST images: one churning high above the north polar
cap at the top of this image, and a second swirling
within and spilling out of the north end of the
giant Hellas impact basin in the lower right portion
of this image. These storms, which are also being
monitored at lower resolution by a growing cadre
of amateur astronomers back on Earth, redistribute
dust and soil around the planet and are responsible
for the changing patterns of the markings seen from
year to year.

These HST images provide high resolution
detail showing how dust is being infilled or eroded
from individual craters, wind streaks, mountain
ridges, and deep valleys. Combined with ground-based
monitoring and similar-scale images obtained concurrently
by the Mars Global Surveyor spacecraft but in different
wavelengths, the HST data will help to constrain
both the composition and mineralogy of the dusty
and darker regions, as well as meteorological models
of the dust storms themselves.

Other prominent features in this
image include an extensive south polar cap (the
whitish region near the bottom) consisting of mostly
dry ice (CO2) that condensed out of the Martian
atmosphere during the long south polar winter (the
image was obtained very close to the first day of
the southern spring season on Mars), a "hood" of
blueish-white water ice clouds at high northern
latitudes (top of image), prominent early morning
(left side) and late evening (right side) blueish-white
water ice clouds forming high up in the extremely
cold Martian atmosphere, and a thin band of water
ice clouds/haze that stretches across the "tropics"
of Mars (across the middle of the image). These
clouds and polar caps are known to wax and wane
as Mars goes through its Earth-like yearly cycle
of seasons. In fact, the equatorial cloud belt is
much less developed than we have seen in previous
HST Mars images, when the planet was much further
from the Sun and thus the atmosphere much colder.
However, the corresponding decrease in water ice
clouds is compensated by the increase in dust storm
activity seen here, since dust storms tend to be
more prominent during southern spring and summer
seasons when the planet is closer to the Sun. These
HST images are helping astronomers fill important
gaps in the seasonal coverage of observations and
models designed to help predict the timing and extent
of future dust storm activities on Mars.

Because of their unique temporal
and spectral coverage, images like these being obtained
from HST are contributing to the selection of landing
sites for the next round of NASA Mars rover missions.
The NASA Mars Exploration Rovers will launch in
2003 and land in early 2004, exploring two different
regions of the surface for at least 90 Martian days
each. Images from HST are helping to define the
most interesting potential landing sites by providing
information on the surface mineralogy and the seasonal/annual
meteorological conditions of dozens of candidate
landing locations.

These spectacular HST images provide
a glimpse of what we can expect to see in 2003,
when Mars makes it closest approach to Earth since
1924 (35 million miles, or 56 million km), and is
closer than it will be again until the year 2287!
During 2003, Mars will be 25% larger in the sky
than during 2001, and surface features as small
as about 7.5 miles (12 km) will be resolvable by
HST. Between now and then, HST and ground-based
observers will continue to monitor the red planet
for new signs of surface changes and meteorological
activity that will further enhance our understanding
of this dynamic world.